In the field of automotive manufacturing, structural components that historically have been fabricated of steel, such as engine cradles, are increasingly being replaced with aluminum alloy castings. Such castings are typically large, convoluted, and relatively thin, and are required to meet the high quality standards of automotive manufacturing. In order to meet these requirements, vacuum-assisted die-casting is typically used to produce such castings.
Vacuum-assisted die-casting machines comprise a piston, sometimes referred to as a “plunger”, which is advanced through a piston bore of a shot sleeve to push a volume of liquid metal into a mold cavity. Vacuum is applied to the piston bore to assist the flow of the liquid metal therethrough. A replaceable wear ring is fitted onto the piston, and makes continuous contact with the inside of the piston bore along the full stroke of the piston for providing a seal for both the vacuum and liquid metal.
For example, FIG. 1 shows a portion of a prior art vacuum-assisted die-casting apparatus, which is generally indicated by reference numeral 20. Vacuum-assisted die-casting apparatus 20 comprises a piston that is moveable within a piston bore 28 defined within a shot sleeve 30 for pushing a volume of liquid metal (not shown) into a die-casting mold cavity (not shown) to form a casting. In the example shown, the piston is positioned at its starting position of the stroke, which is rearward of a port 34 through which the volume of liquid metal is introduced into the piston bore 28.
The piston comprises a piston tip 40 mounted on a forward end of a piston stem (not shown). The piston tip 40 has a front face 42 that is configured to contact the volume of liquid metal introduced into the piston bore 28 via port 34. The piston tip 40 has a wear ring 44 disposed on an outer surface thereof.
In operation, at the beginning of a stroke cycle, the piston is positioned at its starting position in the piston bore 28, and a volume of liquid metal is introduced into the piston bore 28 forward of the piston tip 40 via port 34. The piston is then moved forward through the piston bore 28 to push the volume of liquid metal into the mold cavity for forming a metal casting, and is then moved rearward to its starting position to complete the stroke cycle. During this movement, the wear ring 44 disposed on the piston tip 40 continuously contacts the surface 48 of the piston bore 28, and provides a liquid metal seal for preventing liquid metal from passing between the piston tip 40 and the surface 48 of the piston bore 28. The wear ring 44 also provides a vacuum seal for maintaining vacuum (that is, a low pressure) within the forward volume of the piston bore 28. The cycle is repeated, as desired, to produce multiple metal castings.
As will be understood, the continuous contact between the piston and the piston bore 28 wears the surface 48 of the piston bore over many stroke cycles, and thereby limits the service life of the shot sleeve.
Die-casting shot sleeves having improved wear resistance have been described. For example, U.S. Pat. No. 5,195,572 to Linden, Jr. et al. discloses a two-piece shot sleeve for use with a die casting machine including first and second cylindrical sleeve sections that are removably axially secured together. The sleeve sections are each open at both ends and include an interior passage for the flow of molten metal, and the second sleeve section includes a pour hole for receiving molten metal into the interior passage.
U.S. Pat. No. 5,322,111 to Hansma discloses a lined shot sleeve for use in metal die casting. The lined shot sleeve comprises an elongated main body portion including a first continuous inner wall surface defining a receptacle bore axially extending between a first end and a second end of the main body portion. An elongated ceramic liner is adapted for secure placement within the receptacle bore, the liner including a second continuous inner wall surface defining a cylinder bore axially extending between a first end and a second end of the liner and an exterior wall surface adapted for frictional contact with the first continuous inner wall surface. The ceramic liner acts as a physical and thermal insulator to protect the first continuous inner wall surface of the main body portion from contact with the molten metal.
It is an object at least to provide a novel shot sleeve for a die-casting apparatus and method of fabricating the same.